1. Field of the Invention
The invention relates to a condensation heat exchanger for condensation of nonmetallic vapors, and in particular to a coating of the heat transfer surfaces of the condensation heat exchanger. The coating is used to extend the service life of the cooling tubes and to improve the heat transfer at the heat transfer surfaces.
2. Discussion of Background
In condensation heat exchangers, the lifespan of the heat transfer surfaces plays an important role, since damage to the heat transfer surfaces causes the entire installation in which the condensation heat exchanger is installed to fail. The state of the heat transfer surfaces of condensation heat exchangers is adversely affected, inter alia, by drop impingement erosion and corrosion. Damage caused by drop impingement erosion occurs in particular at those heat transfer surfaces which are exposed to a high-speed flow of steam. There, drops which are present in the steam which is to be condensed impinge on the heat transfer surfaces, energy being transferred to the surface by the impact or by shear forces. Erosion occurs if, in the event of very frequent drop impingement, the energy transferred is sufficient to plastically deform the surface material, leads to creep in the case of ductile materials or leads to intercrystalline fatigue failure in the case of hard materials.
With steam condensers in steam power plants, it has been observed that relatively large drops with diameters in the region of 100 μm and velocities of 250 m/s cause drop impingement erosion. In particular the cooling tubes at the periphery of a tube bundle are effected, while the tubes in the interior of a tube bundle remain protected from direct drop impingement erosion.
The occurrence of drop impingement erosion is highly dependent on the materials properties, such as hardness, ductility, elasticity, microstructure and roughness, materials made from titanium and titanium alloys being distinguished by a certain resistance to erosion, although this resistance is insufficient and predominantly due to their high hardness. In the case of steam condensers used in steam power plants, drop impingement erosion of this type is inhibited by a suitable selection of material for the cooling tubes, such as for example for stainless steel, titanium or chromium.
Furthermore, drop impingement erosion presents a problem in particular at low condenser pressures and therefore relatively high vapor velocities, as for example in steam condenses in steam power plants which are operating at part loads. When steam condenses on heat transfer surfaces, according to the prior art a film of condensate which spreads over the entire surface is formed. This film of condensate increases the overall thermal resistance between steam and cooling liquid flowing in the tubes, with the result that the heat transfer capacity is reduced. For this reason, there have long been efforts made to provide heat transfer surfaces with a coating which, by dint of hydrophobic properties, prevent the formation of a film of condensate, so that drop condensation occurs at the surface. The formation of drops allows the condensate to run off more quickly than if a film is formed. As a result, the surface of the heat exchanger is cleared, so that vapor can condense again at the surface without being impeded by a film of condensate. The overall heat resistance therefore remains relatively low. By way of example, layers of Teflon or enamel have been tried for this purpose, but without great success, since these layers had little resistance to erosion and corrosion.
In terms of the coating, it is important to solve the problem of the resistance to erosion and corrosion and also that of the bonding of the coating to the heat transfer surfaces. These problems need to be solved in particular in view of the desired long operating time of the condensation heat exchanger, such as for example in the cooling tubes of a steam condenser, which have to be able to operate for a period of several years.
An example of a coating is disclosed by WO 96/41901 and EP 0 625 588. These documents describe a metallic heat transfer surface with what is described as a hard-material layer comprising plasma-modified amorphous hydrocarbon layers, also known as diamond-like carbon. Amorphous carbon is known for its elastic, extremely hard and chemically stable properties. The wetting properties of the hard-material layer of amorphous carbon are altered by the incorporation of elements such as fluorine and silicon, in such a manner that the layer acquires hydrophobic properties. For bonding to the substrate, an interlayer is applied between the substrate and the hard-material layer, the transition from the interlayer to the hard-material layer being produced by means of a gradient layer. However, the hard-material layer is ultimately only resistant to erosion on account of its inherent hardness.
DE 34 37 898 has described a coating for the surfaces of a heat exchanger, in particular for the surfaces of condenser cooling tubes, comprising a triazine-dithiol derivative. This layer material effects drop condensation and therefore improves the heat transfer. Furthermore, the coating is distinguished by good bonding to the cooling tubes.
DE 196 44 692 describes a coating comprising amorphous carbon which brings about drop condensation on the cooling tubes of steam condensers. The surface of a cooling tube is roughened prior to the application of the amorphous carbon, with the result that the effective interface between the cooling tubes surface and the coating is increased. As a result, the heat resistance between coating and base material is reduced. After the coating, the surface is smoothed, so that coated and uncoated regions are formed next to one another.